Cathode ray tubes (CRT's) commonly used in color television and related display applications conventionally utilize unitized electron gun assemblies which direct a plurality of controlled electron beams to the display screen of the tube. In certain gun assembly constructions, the first and second grid electrode components, such being normally control and screen grid electrodes, are often formed as substantially planar members oriented in substantially parallel planes in spaced apart superposed relationship. In multi-beam guns each of these first and/or second planar electrodes contains several spatially related apertures to accommodate the respective electron beams generated within the assembly. It is very important that these several apertures be accurately and consistently spaced relative to the related apertures in the adjacent electrode components, and, in the case of the first electrode component, with the respective cathode surfaces from which the specific electron beams emanate. These and associated electrodes are conventionally affixed to at least two longitudinal insulative support members of the integrated gun assembly by supporting projections extending from the respective electrode components.
Fabrication of the gun assembly involves embedment of the supporting projections of the related electrode components into the temporarily heat-softened longitudinal insulative support members. In this operation, which is commonly referred to as "beading", the softened support members on opposed sides of the assembly are pressured inward toward the several electrode components thereby forcing the supporting projections thereof into the support members. The opposing compressive pressures tend to exert a distorting force upon the electrode components, this being especially critical to the planar components wherein a bowing or arcuate bending effect sometimes results. Such bowing, however slight, changes the aperture locations relative to those in the adjacent electrode components, thereby producing deleterious inter-electrode spacing relationships within the gun structure. These uncontrollable changes in the related aperture spacings are particularly troublesome in in-line gun constructions wherein the first and the second grid electrodes usually have related apertures of small diameter and close spacings.
Two serious manufacturing control problems are caused by the bowing or warping of the first (G1) and/or second (G2) electrode components. The first of these is variation of cutoff and associated cutoff ratio. Cutoff is defined as the positive cathode (K) voltage at which the electrons cease to flow through the G1 aperture. Cutoff ratio is the ratio of the highest cutoff voltage to the lowest cutoff voltage of the three guns in a given tube. Cathode cutoff ratio is now commonly specified at 1.25, a condition which requires precise G1, G2, and K-G1 spacing control. This has proven to be one of the more difficult manufacturing control problems.
The second control problem relating to bowed G1 and G2 electrodes is variation of focus quality. This is largely determined by gun design, but for the gun construction to be successful, three factors are essential: (a) high quality parts must be used, (b) parts alignment must be accurately maintained, and (c) K-G1 and G1-G2 spacings must be precisely controlled at or near design center for optimum focus performance. This factor is directly related to bow-free electrodes. The most difficult production control parameter is the endeavor to achieve consistent K-G1 spacings for the three associated beams.
There are disclosures in the prior art to ruggedize in-line planar type electrodes by incorporating strengthening ribs such as those taught by Floyd K. Collins in U.S. Pat. Nos. 4,049,990 and 4,049,991.
A second grid electrode having channels therein is also shown in the gun structure disclosed by Allen P. Blacker and James W. Schwartz in U.S. Pat. No. 4,058,753.
While teachings of incorporating strengthening ribs fulfilled the existing needs at the time of disclosure, the state of the CRT art has advanced to stages of greater constructional sophistication wherein gun assemblies are made smaller and more compact, and tube operating requirements more stringent and exacting. In view thereof, improved strengthening of planar type electrodes, to prevent bowing during tube fabrication, is essential to achieving the desired tube performance characteristics required in the present state of the art.